1. Field of the Invention
The present invention relates generally to optical fibers suitable for use in sensing applications in harsh environments.
2. Technical Background
Optical fiber has become a favorite medium for telecommunications due to its high transmission capacity and immunity to electrical noise. Over the past decade, optical fibers have also been used in point and/or distributed sensing applications. Fiber has been used in oil and gas industries to provide critical information for oil exploration, well drilling and production. In these oil/gas wells, optical fibers are used as distributed sensors to monitor/gauge temperature, pressure, and flow information along the depth of geophysical wells. However, the harsh down-hole environment presents a severe reliability challenge. In a typical down-hole environment, optical fiber experiences high temperature (up to 300° C.), high pressure (up to 1000 atm), moisture, hydrogen and other harmful species such as CO2, and H2S.
Specialized fiber coating designs have been developed to protect the optical fibers used in such harsh environments. For example, amorphous carbon-based thin coating (so called “hermetic coating”) and metal coatings have been used. However, there has not much work done in the area of the composition of silica glass in the fiber other than using either pure silica core fibers with a Fluorine-doped cladding, or, more typically, fibers with cores consisting of Ge doped silica.
Hermetic coating provides a protective layer which prevents ingress of molecular water or hydrogen into silica glass of the fiber. Hermetic coating also enables highly reliable deployment of the fiber under smaller coil diameters. The presence of hermetic coating provides the optical fiber with improved mechanical integrity. Ge doped fibers have an absorption peak in the visible and near-IR wavelength. Furthermore, our recent studies revealed that applying a hermetic coating onto GeO2-doped fibers completely hinders H2 ingression into fiber core for temperatures up to 150° C., but not above 170° C. For example, elevated attenuation peak at 1240 and 1381 nm and overall elevation of background loss is observed. This indicates that the hermetic layer is no longer genuinely hermetic at temperatures above 170° C. Pure silica core fibers with hermetic coatings also experience loss increase at temperatures above 170° C. and may start having reliability issues at these temperatures.